BAGHDAD — Iraq's prime minister urged al-Qaida-linked fighters who have overrun two cities west of Baghdad to give up the battle, vowing Wednesday to press forward with a push to regain control of the mainly Sunni areas.

The United Nations, meanwhile, warned that the area in Anbar province is facing a "critical humanitarian situation" as food and water supplies are starting to run out.

Sectarian tensions have been rising in Iraq for months as minority Sunnis protested what they perceive as discrimination and random arrests by the Shiite-led government. But violence spiked after the Dec. 28 arrest of a Sunni lawmaker sought on terrorism charges and the government's dismantling of a months-old anti-government Sunni protest camp in the Anbar provincial capital of Ramadi.

As clashes broke out, al-Qaida-linked gunmen assaulted Ramadi and nearby Fallujah, cities that were among the bloodiest battlefields for U.S. forces during the war. The militants overran police stations and military posts, freed prisoners and set up their own checkpoints.

The United States and Iran have offered materiel help for the Iraqi government but say they won't send in troops.